Abstract

This report presents a preliminary map and database of
Quaternary deposits and liquefaction susceptibility for the nine-county San
Francisco Bay region, together with a digital compendium of ground effects
associated with past earthquakes in the region. The report consists of (1) a
spatial database of fivedata layers (Quaternary deposits, quadrangle index, and
three ground effects layers) and two text layers (a labels and leaders layer
for Quaternary deposits and for ground effects), (2) two small-scale colored
maps (Quaternary deposits and liquefaction susceptibility), (3) a text
describing the Quaternary map, liquefaction interpretation, and the ground
effects compendium, and (4) the databse description pamphlet. The nine counties
surrounding San Francisco Bay straddle the San Andreas fault system, which
exposes the region to serious earthquake hazard (Working Group on California
Earthquake Probabilities, 1999). Much of the land adjacent to the Bay and the
major rivers and streams is underlain by unconsolidated deposits that are
particularly vulnerable to earthquake shaking and liquefaction of
water-saturated granular sediment. This new map provides a modern and regionally
consistent treatment of Quaternary surficial deposits that builds on the
pioneering mapping of Helley and Lajoie (Helley and others, 1979) and such
intervening work as Atwater (1982), Helley and others (1994), and Helley and
Graymer (1997a and b). Like these earlier studies, the current mapping uses
geomorphic expression, pedogenic soils, and inferred depositional environments
to define and distinguish the map units. In contrast to the twelve map units of
Helley and Lajoie, however, this new map uses a complex stratigraphy of some
forty units, which permits a more realistic portrayal of the Quaternary
depositional system. The two colored maps provide a regional summary of the new
mapping at a scale of 1:275,000, a scale that is sufficient to show the general
distribution and relationships of the map units but cannot distinguish the more
detailed elements that are present in the database. The report is the product of
years of cooperative work by the USGS National Earthquake Hazards Reduction
Program (NEHRP) and National Cooperative Geologic Mapping Program, William
Lettis and & Associates, Inc. (WLA) and, more recently, by the California
Division of Mines and Geology as well. An earlier version was submitted to the
Geological Survey by WLA as a final report for a NEHRP grant (Knudsen and
others, 2000). The mapping has been carried out by WLA geologists under
contract to the NEHRP Earthquake Program (Grants #14-08-0001-G2129,
1434-94-G-2499, 1434-HQ-97-GR-03121, and 99-HQ-GR-0095) and with other limited
support from the County of Napa, and recently also by the California Division
of Mines and Geology. The current map consists of this new mapping and
revisions of previous USGS mapping.